Radiation therapy is used to treat cancers and other conditions in patients. About half of all cancer patients receive some type of radiation therapy sometime during the course of their treatments. One commonly used form of radiation therapy is external beam radiation therapy. In external beam radiation therapy a high-energy, x-ray beam generated by a machine, usually a linear accelerator (linac), a gamma-ray beam emitted from an isotope, or charged particles generated from a particle accelerator is/are directed at a tumor or cancerous cells (i.e., the “target”) inside the patient's body. While the radiation kills the cancerous cells, it also harms normal tissue and organs in the vicinity of the tumor/cancerous cells in the patient. Thus, the goal in radiation therapy is to deliver the required dose of radiation to the target volume, while minimizing the radiation dose to surrounding normal tissue that may cause complications and harm to the patient.
Before a patient is treated with radiation, a radiation treatment plan must be developed through a process called “treatment planning,” which begins with simulation. During simulation, detailed imaging scans show the location of a patient's tumor and the normal areas around it. These scans are usually performed using computed tomography (CT), but they also can be performed using magnetic resonance imaging (MRI), x-rays or ultrasound.
The ability of radiation therapy to achieve the goal of tumor eradication and normal tissue sparing depends on the degrees of freedom provided by the radiation delivery machine and on the physics of dose deposition. These freedoms and physics principles are incorporated in the treatment planning process.
A common type of external-beam radiation therapy is called three-dimensional conformational radiation therapy (3D-CRT). 3D-CRT allows the radiation beams to be shaped from a limited number of fields to conform to the beam's eye-view of the target area.
Intensity-modulated radiation therapy (IMRT) provides more freedom than 3D-CRT by allowing the intensities of the radiation beams to vary within a radiation field in addition to field shaping. The goal of IMRT is to increase the radiation dose to the areas that need it and reduce radiation exposure to specific sensitive areas of surrounding normal tissue. The treatment planning system optimizes the beam intensity distribution to achieve maximally this goal. Compared with 3D-CRT, IMRT can reduce the risk of some side effects, such as damage to the salivary glands (which can cause dry mouth or xerostomia), when the head and neck are treated with radiation therapy (Veldeman et al., “Evidence behind use of intensity-modulated radiotherapy: A systematic review of comparative clinical studies,” Lancet Oncology 9(4): 367-375 (2008); and Erratum in: Lancet Oncology 9(6): 513 (2008)).
Tomotherapy (Detorie, “Helical Tomotherapy: A new tool for radiation therapy,” J. Amer. Coll. Radiol. 5(1): 63-66 (2008)) and intensity-modulated arc therapy (IMAT) (Yu, “Intensity modulated arc therapy using dynamic multi-leaf collimation: An alternative to Tomotherapy,” Phys. Med. Biol. 40(9): 1435-1449 (1995)) are IMRT deliveries in rotational forms. In tomotherapy the patient is translated linearly as the source of radiation is making circular movements, thereby the relative motion of the radiation beam and the patient is a helix. Because the gantry on which the linear accelerator is mounted can only rotate in a single transverse plane, such “coplanar” rotational IMRT methods limit the range of beam directions available to create an optimal plan. Consequently, these techniques have not been shown to create significantly better dose distributions than IMRT with fixed beams.
3D-CRT and IMRT are typically delivered using a linear accelerator mounted on a C-arm gantry (as shown in FIG. 1) or a ring-like gantry, which is capable of only single plane rotation. These methods have limited ability to deliver high radiation doses in a single session or a few sessions without exceeding the tolerance of surrounding organs and tissues.
Stereotactic radiosurgery (SRS) and stereotactic body radiation therapy (SBRT) deliver one or more high doses of radiation to a small tumor (R. Timmerman and B. Kavanagh, “Stereotactic body radiation therapy,” Curr. Probl. Cancer 29: 120-157 (2005)). SRS is commonly used for treating intracranial lesions and requires the use of a head frame or other device to immobilize the patient during treatment to ensure that the high dose of radiation is delivered accurately. The Gamma Knife (Bhatnagar et al., “First year experience with newly developed Leksell Gamma Knife Perfexion,” J. Med. Phys. 34(3): 141-148 (2009)) is a dedicated SRS system for treating intracranial lesions. Gantry-based linear accelerator systems are also used for SRS. Both allow radiation beams to be incident on the target from directions outside the transverse plane. SBRT is used to treat tumors that lie outside the brain. SBRT is usually given in more than one treatment session. Methods of extending the Gamma Knife concept to the rest of the body are also proposed, such as with the GammaPod system for the treatment of breast cancer (Yu, et al., “GammaPod—A new device dedicated for stereotactic radiotherapy of breast cancer,” Med. Phys. 40(5): 1703 (2013)) and the use of multiple sources mounted on an arc element that rotates (Pastyr et al., U.S. Pat. No. 6,259,762 B) for treating tumor sites other than in the brain. The principle of SRS and SBRT is geometric focusing of the beams to create a high dose within the target volume with a fast fall off of dose outside this volume. Focusing is achieved by aiming the radiation beams at the target from hundreds or thousands of directions. However, the ability to modulate the shape and intensity of these beams is limited. As such, SRS and SBRT have limited ability to spare surrounding tissues while maintaining a high and uniform dose within the target volume. For example, although the modern Gamma Knife has the ability to reach slightly below the base of the skull, attempts to use the Gamma Knife for treating complex targets in the head and neck region have had limited success. These regions (spinal cord, parotid glands, mandible, etc.) have complex geometric relationships to the target, and all have different radiation tolerances that need to be respected.
Techniques for delivering intensity modulated radiation from a large number of beam angles have been proposed. The CyberKnife system (J. Adler, “CyberKnife radiosurgery for brain and spinal tumors,” International Congress Series 1247: 545-552 (2002)) employs a linear accelerator mounted on a robotic arm. It can deliver radiation from a large number of non-coplanar angles, but the practical number of beam angles is limited by the long treatment times associated with a large number of independent beams. Furthermore, the range of beam directions from the posterior hemisphere of the patient is restricted because of geometry constraints. Furthermore, the degree of beam modulation is limited by its collimator design.
Maurer and colleagues at Accuray, Inc., have proposed a number of alternative solutions using a fixed ring gantry, rather than a robotic arm (U.S. Pat. App. Pub. No. US 2011/0210261 A1; U.S. Pat. App. Pub. No. US 2011/0301449 A1; and U.S. Pat. App. Pub. No. US 2012/0189102 A1). While ring gantries are desirable for diagnostic imaging, where a single transverse plane or limited non-coplanar angles are used for the imaging beams, they are not ideal for treatment where a larger range of non-coplanar angles is desirable. For radiation treatment of most anatomical sites, the radiation beams are preferably directed to the target from one side of the patient's transverse axis, often from a large angle relative to this axis. For example, in treating intracranial lesions, most beams should be directed from the upper hemisphere (above the top of the patient's head) rather than from the lower hemisphere. Furthermore, it is often advantageous to use beams that are directed almost along the patient's longitudinal axis, demanding highly non-coplanar beams. In treating prostate cancer, it is generally preferable to direct beams from the lower body, rather than from the upper body, because it is better to have the beams go through less tissue and critical structures in the abdominal region. Flexible beam orientation ability throughout the lower body and some of the upper body is needed to achieve an optimal plan. The ring gantry systems proposed by Maurer and colleagues have limited ability to take advantage of such anatomical preferences or achieve highly non-coplanar beam directions.
Alternatively to exploring additional degrees of freedom, the physics of dose deposition can be altered by using different forms of radiation. External-beam radiation therapy can be delivered by proton beams and other charged particle beams. The charged particle beams differ from photon beams mainly in the way they deposit energy in living tissue.
Whereas photons deposit energy in small packets all along their path through tissue, including in regions both proximal to and distal to the target volume, protons deposit much of their energy at the end of their path (called the Bragg peak) and deposit less energy along the way. Proton energy deposition can thus be tailored to be largely within the target volume. The main limitation in providing these proton beam and charged particle treatment facilities is the extremely high cost.
Most existing linear accelerators or teletherapy machines can rotate around an axis by the rotation of the gantry on which the source of radiation is mounted. See, for example, FIG. 1, which is a drawing of a basic structure of a typical radiation treatment system in which a radiation-emitting head is mounted on a rotatable C-arm gantry. The locus of the radiation source forms a circle. During gantry rotation, the radiation beam is pointed at the rotational center, commonly referred to as the “isocenter.” This design limits the beam directions to mostly planar angles and, therefore, limits the quality of treatment plans achievable with high-energy photon beams.
The present disclosure seeks to overcome the limitations of the attendant systems and methods currently available in the art by providing, among other things, a method to allow radiation beams to be focused from a broad solid angle by combined longitudinal and latitudinal rotations of the radiation source. In view of the foregoing, the present disclosure describes a method and a radiation delivery system to increase further the utility and clinical efficacy of photon-based treatment systems via increasing the degrees of freedom in beam delivery beyond that achievable with existing IMRT and SRS/SBRT systems. Specifically, this is achieved by allowing intensity-modulated photon beams to be delivered from a very large number of beam directions, including those which are highly non-coplanar. The solid angle range includes all longitudinal angles (about the patient's longitudinal axis) and a broad range of latitudinal angles. The methods and system combine, in a practical design, the geometric focusing of SRS/SBRT and intensity modulation of IMRT, thereby providing capabilities not attainable by either IMRT or SRS/SBRT alone. This and other objects and advantages, as well as inventive features, will become apparent from the detailed descriptions provided herein.